Gary, it appears that you may be a bit too concerned about this. Any significant degree of oxidation to the exposed ends may take years and is easily cleaned or trimmed off. The portion of the wire that's covered by the jacket is essentially sealed from contact with the air. Cutting open wire that's been around for many years(e.g., my wire about 20 years old mentioned above)for a new connection reveals clean bright copper in the newly cut ends. If you have any old wires around, you can examine the stripped ends for oxidation or cut a new section to see how the previously covered wire is.
